Antenna tuning system



May 25, 1954 M. l. .JACOB ET'AL 2,679,581

ANTENNA TUNING SYSTEM l Filed. May 2o, 195o.v

F.M. Tronsmiter I. Modulation 4 L Source i v v y l lol n E E i E A'A'A'A'A'A A'AVA'A A'AVA AVL AVA l A 1; 24 22 2 2625; T

C l'l'l'l'l'g'l'lI WITNESSES: INVENTORS T iv/W RobertJ.Howe-Hond Mork I.Jocob. 72M a, i .Bfwl

ATTORNEY Patented May 25, 1954 2,679,581

UNITED ANTENNA TUNING SYSTEM Mark I. Jacob, Baltimore, and Robert J. Howell, Catonsville, Md., assignors to Westinghouse Electric Corporation, lEast Pittsburgh, Pa., l`a corporation of Pennsylvania Application May 20, 1950, Serial No. 163,128

Claims. (Cl. Z50-17) 1 2 This invention relates to low frequency, fremitter frequency below this range, another caquency modulated, radio transmitters, and relates pacitor is switched across the antenna circuit, more particularly to circuits for tuning the antuning it to a lower frequency f2 which equals tenna circuit of such a transmitter to the instanwhich equals fi-Af. The antenna will remain taneous transmitter frequency. 5 at the resonant frequency f2 only while the trans- For such duties as military communications mitter is Within the range where dependability is of prime importance, very low frequencies of the radio spectrum are used fa gf since the ground waves in which the energy is 2 transmitted are but slightly subject to fading and to daily and seasonal variations. The main and 5 0 0n 1n suon a' mannn' that the antenna difficulties encountered in the use of such fre- @mams tuned to the transmtter frequency With' quencies have involved the high Q factor for m the range antennas of reasonable size. This is for the reason that when the frequency of a driving signal l5 A f is suddenly changed, the circulating current in 2 a driven circuit consists of a signal at the original frequency which is decaying exponentially from the moment of frequency change, and a signal at the new frequency which builds up exponen- 2O tially at the same moment. In high Q, low frequency circuits, the time required for new equilibrium conditions to be established following each frequency change, may be so long as to N=the number of capacitors to be switched in Preferably, the capacitors are connected in and out of the antenna circuit by thyratrons which are caused to conduct by sharp cut-off, pentode control tubes, each of which responds to a different level, signal voltage.

In one embodiment of the invention, where approach the modulation rate, or in frequency and out ofthe antenna circuit.

Shlft keyed te1 egraph Sylstfms the um@ of a f0=the center frequency of the transmitter and mark or space signal, requiring a decrease 1n the the antenna resonant frequency modulation rate, or in the keying speed, for n avoiding distortion in the transmitted signals. Af=tne lncemental Change 0f antenna resonant Such diiculties may be overcome by mainfrequencytaining the antenna resonant frequency the same as the instantaneous transmitter frequency as broadly disclosed in the C. E. McClellan applica- A e=the instantaneous modulation signal voltage.

tion, Serial No 58,064, med November 3 1948- Ae =the deflection sensitivity of the transmitter, and

This invention provides for maintaining an antenna of a transmitter at the instantaneous eg=tne mld-901m Qf a' range 0f gld VOltge transmitter frequency by switching a series of neCeSSa 1`Y t0 Shlft the @91'113101 tubes from capacitors in and out of the antenna circuit so Satnlatlon t0 Cut-01T: T1115 falls? 1S a Small that each additional capacitor added to the cir- Dortlon 0f the maxlmllm ValatlOYl 1n glld cuit lowers the resonant frequency of the antenna 40 VOlageby a definite frequency increment Af, and that each capacitor removed from the circuit increases the resonant frequency by the Same contrai tube that its cathode is negative and its inrementi' The Signal modulating the trans' Plate is positive with respect to the thyratron mitter raises and lowers the frequency of the cathoda When a pentode draws Saturation transmltter above or below .demte Center fre' plate current, the control grid of its associated quency aim cqntrols the swltchmg ,Sequence of thyratron is suiiiciently negative to cut the thyrathe capacitors 1n such a manner that the antenna tron om and when a pentode is cnt off Sufficient 1s hem at a' demte resonant frequency f1 time positive voltage is applied to the control grid of the. msftantaneous frequency of the transmltter its associated thyratron to cause it to conduct.

vanas m the range For N stages, where N is an even integer, the A f fixed bias applied to the nth control tube will be 2 e -l--I- Z`n Ae When the modulating signal lowers the transg 2 y2 The capacitors are each connected in series with a thyratron which is so connected to a pentode fii The signal voltage e is applied in series with a fixed bias voltage to the pentode grids. A signal voltage of polarity and magnitude sufficient to cause the transmitter frequency to vary in the range inici, will, if the antenna resonant frequency is above or below fn, cause a capacitor to be switched in or out of the antenna circuit for maintaining the antenna resonant frequency at fn.

Signal distortion introduced by this system is a function of the number of stages used. Experiments so far have indicated that for sine wave modulation signals up to 25% distortion is present in three steps, distortion is present in seven steps, and 7 distortion is present in eleven steps. Voice transmission is excellent when volume compression of the voice signal is employed so that low level but high intelligence signals are suiiciently large to cause the resonant frequency of the antenna circuit to be shifted. Facsimile signals can be transmitted with excellent results, best results being obtained by adjusting the stepped antenna resonant frequencies to correspond to the instantaneous transmitter frequency which represents the signal levels of the picture shadings desired. Frequency shift keying can be satisfactorily accomplished.

Among the advantages of this invention is that its capacitive method of tuning is more efcient than inductive methods. Capacitors having less loss than transformers can be more readily obtained. The capacitor type of circuit is simpler and in less susceptible to transients caused by the sudden firing of the thyratrons.

An object of this invention is to maintain a tuned circuit at the frequency of a variable frequency driving signal by switching capacity in and out of the circuit.

Another object of this invention is to vary the resonant frequency of a tuned circuit in accordance with changes in the frequency of a driving signal by switching capacity in and out of the circuit.

Another and more specific object of this invention is to maintain an antenna of a frequency modulated, radio transmitter tuned to the instantaneous frequency of the transmitter by switching capacitors in and out of the antenna circuit.

The invention will now be described with reference to the drawing, the figure of which is a circuit schematic of one embodiment thereof.

The modulation source II) is connected to the frequency modulation transmitter I I, which is coupled through the coupling inductor I2 to the inductor I3 of the antenna circuit I4. The inductor i3 is tuned by the capacitor I5 which is connected at one side to one end of the inductor I3, and is connected at its other side through the resistance I6 to the other end of the inductor I3, and to ground. The antenna circuit I4 is a high Q, relatively low frequency and relatively high .power circuit.

The modulation source IIJ is also connected to the primary winding of the transformer I1, the secondary winding of which is connected at one side to the interconnected cathodes of the pentode, sharp cut-off, control tubes I8, I9, 20 and 2I, and at its other side to the junction point of the series-connected resistors 22 and 23, which are connected in series with the resistors 24 and 25, which in turn are connected through the resistors 25 and 21 to the terminals of the negative bias voltage source 28 illustrated symbolically as a battery. The instantaneous modulation signal voltage e is applied to the primary of the transformer I1 and a voltage e' appears across its secondary.

The control grid of the tube I8 is connected through the grid resistor 23 to the junction point of the resistors 24 and 2G. The control grid of the tube I9 is connected through the grid resistor 3G to the junction point of the resistors 24 and 22. The control grid of the tube 20 is connected through the grid resistor 3| to the junction point of the resistors 23 and 25. The control grid of the tube 2| is connected through the grid resistor 32 to the junction point of the resistors 25 and 21.

The plate of the tube I8 is connected through the load resistor 33 to a maximum voltage, positive terminal of the plate voltage supply source 34, and to the grid of the thyratron 35.

The cathode of the thyratron 35 is grounded, and its plate is connected through the capacitor 35 to the ungrounded end of the antenna inductor I3. The cathode of the gas-filled diode tube 31 is connected to the plate of the thyratron 35, and its `plate is grounded.

The plate of the tube I9 is connected through the load resistor 38 to the maximum voltage, positive terminal of the source 34, and to the grid of the thyratron 39.

The cathode of the thyratron 39 is grounded, and its plate is connected through the capacitor 40 to the ungrounded end of the antenna inductor I3. The cathode of the gas-filled diode tube 4I is connected to the plate of the thyratron 39, and its plate is grounded.

The plate of the tube 20 is connected through the load resistor 42 to the maximum voltage terminal of the source 34, and to the grid of the thyratron 43.

The cathode of the thyratron 43 is grounded, and its plate is connected through the capacitor 44 to the ungrounded end of the antenna inductor I3. The cathode of the gas-filled diode tube 45 is connected to the plate of the thyratron 43, and its lplate is grounded.

The plate of the tube 2| is connected through the load resistor 46 to the maximum voltage terminal of the source 34, and to the grid of the thyratron 41.

The cathode of the thyratron 41 is grounded, and its plate is connected through the capacitor 4B to the ungrounded end of the antenna inductor I3. The cathode of the gas-filled diode tube 49 is connected to the plate of the thyratron 41, and its plate is grounded. v

The plate circuits of the thyratrons are supplied with alternating current through their connection to the antenna inductor I3. They conduct when their grids are suiciently positive and their plates are positive. The diode tubes 31, 4I, 45 and 49 serve to discharge the tuning capacitors 36, 40, 44, and 48 by conducting on the negative half cycle which permits the deionization of the thyratrons to which they are connetted after firing.

The suppressor grids of the tubes|8,i9,2iand 2l are tied to their respective cathodes, and their screen grids are interconnected and connected to an intermediate voltage terminal of the source the source is being grounded atl a point between that terminal and the maximum voltage terminal, whereby the screen grids of the tubes i8, i9, 2% and 2i are positive with respect to their cathodes which are connected to the negative terminal of the source 33, and are negative with respect to ground.

The cathodes of the control tubes l il, i9, 29 and 2| are negative, and their plates are positive with respect to the corresponding thyratron tubes 3.5, 35, i3 and [il respectively. Thus, when a control tube draws saturation current, the voltage drop at its plate lowers the grid of the corresponding thyratron to the cut-off voltage of the thyratron, when a control tube is cut-off, the increase in voltage at its plate provides sufficient positive grid voltage to the corresponding thyratron to cause it to conduct.

When the thyratron 35 conducts, it acts as an electronic switch connecting the capacitor 3 to ground and therefore, connecting its across the antenna inductor i3, thereby lowering the resonant frequency of the antenna. Likewise, when the thyratrons 39, i3 and lll conduct, they act to connect the capacitors ri, :irl and 65S, respectively, across the inductor i3, each additional capacitor so connected, decreasing the resonant frequency of the antenna.

The control grids of the tubes i8, IQ, 2t and ZI are biased to diderent potentials by the bias voltage source 23. The control grid tube it is biased by a voltage E more negatively than that of the tube lil, the control grid of the tube i@ is biased by the same voltage E more negatively than that of the tube 2li, and the control grid of the tube 2s biased by the same voltage E' more negatively than that or" the tube 2l.

The bias voltage for the tube lil is cg-l-S/Zne; the bias volta-ge for the tube i3 is eg-l-l/Zne; the bias voltage for the tube E23 is eg--l/Zne; and the bias voltage for the tube 2i is eg-S/Zne where eg is the grid bias point where the tubes go from cut-oif substantially to saturation, and ne is the incremental change of modulation signal voltage.

Thus, the tubes i8 and lil are biased different voltages below cut-olf, while the tubes 2G and 2i are biased different voltages above cut-off. IThe thyratrons and :lil associated with the control tubes lil and i9 normally conduct while the thyratrons [le and lll' associated with the control tubes 2e and 2i normally are non-conducting. The capacitors ii and #lil associated with the thyratrons t5 and normally are connected across the inductor i3, while the capacitors 44 and 48 associated with 'the thyratrons i3 and 4l normally are disconnected from across the inductor i3.

The modulation source it applies signal voltages through the transformer il between the cathodes of the control tubes i8, ES, 2li and 2l and their control grids, which is in series with the bias voltages. At a given modulation signal reference level, which for the purpose of illustration may be assumed to be the zero modulation level, at which level the transmitter operates at its assigned center frequency, the tubes 2li and Zi conduct, while the tubes if and id are non-conducting. A rst predetermined increase in positive signal voltage above the zero level will cause the control 'tube i9 which is biased the least below cut-off, to conduct. This will result in an increase inthe voltage-at itsplate and at the grid of its associatedthyratron 39, causing the thyratront' to cease conducting whenits plate goes negative, and to disconnect its associated capacitor 48 from the antenna circuit, thereby raising the antenna resonant frequency. A further predetermined increase in positive signal voltage will cause the control tube I8 to conduct, causing the grid of its associated thyratron 35 to become sufiiciently less negative to cause the thyratron 35 to cease conducting, when its plate goes negative, and to disconnect its associated capacitor 35 from the antenna circuit, thereby further raising the antenna resonant frequency.

A first predetermined increase in negative signal voltage will cause the control tube 2@ which is biased the leastabove cut-off, to cut oif, the resulting increase in its plate voltage and in the grid voltage of the thyratron tube 43 causing the latter to conduct when its plate is positive, and to connect its associated capacitor lll in the antenna' circuit, thereby lowering the antenna resonant frequency. A further predetermined increase in negative signal voltage will cause the control tube 2! to cut off, the resulting increase in its plate voltage and in the grid voltage of its associated thyratron d'1, causing the latter to conduct when its plate is positive, and to connect its associated capacitor d3 in the antenna circuit, thereby further lowering the antenna resonant frequency.

At the same time the modulation source supplies signal voltages to the control tubes, it frequency modulates the transmitter i i, raising and lowering its instananeous frequency. The constants of the circuit components are so chosen that, as referred to previously, each additional capacitor added to the antenna circuit lowers its resonant frequency by a definite frequency increment Af, and the antenna is maintained at a definite resonant frequency f1, while the instantaneous frequency of the transmitter varies in the range fr f-Af/ 2. When the modulating signal lowers the transmitter frequency below this range, another capacitor is switched acrossthe antenna circuit, tuning it to a lower frequency f2, where f2=f1-Af.

The antenna resonant frequency f2 will be maintained only while the transmitter operates in the range fyi-Af. When the modulating signal lowers the transmitter frequency below this range, another capacitor is shunted across the antenna circuit, tuning it to a lower frequency fs, where f3=fz-Af.

The antenna resonant frequency f3 will be maintained only while the transmitter operates in the range fai-Af. When the modulating signal lowers the transmitter frequency below this range, another capacitor is shunted across the antenna circuit, tuning it to a lower frequency f4, Where 4=f3-A.

Likewise, when the modulating frequency raises the transmitter frequency, first one and then another capacitor will be switched out of the antenna circuit, raising its resonant frequency, first from f4 to fs, then from f3 to f2, then from f2 to f1. More or fewer steps than four could, of course, be provided, according to this invention, depending upon the duty involved.

We claim as our invention:

1. Frequency modulation apparatus comprising a frequency modulation transmitter, an antenna circuit for said transmitter, said circuit including an inductor, a capacitor for lowering the resonant frequency of said circuit when connected across said inductor, an electron tube connected to said inductor and said capacitor for electrically connecting said capacitor across said inductor upon conduction of said tube, a modulation signal source for said transmitter, and means responsive to signals from said source for causing said tube to conduct when said signals cause the frequency of said transmitter to decrease.

2. Frequency modulation apparatus comprising a frequency modulation transmitter, an antenna circuit for said transmitter, said circuit including an inductor, a plurality of capacitors for connection across said inductor for lowering the resonant frequency of said circuit, a plurality of electron tubes, one of said tubes being connected to each of said capacitors and to said inductor for electrically connecting each capacitor across said inductor upon conduction of its associated tube, a modulation signal source for said transmitter, and means responsive to signals from said source for causing said tubes successively to conduct when the signals lower the frequency of said transmitter, and for varying the number of tubes which conduct, in accordance with the magnitude of said signals.

3. Frequency modulation apparatus comprising a frequency modulation. transmitterfan antenna circuit for said transmitter, said circuit including an inductor, a capacitor for lowering the resonant frequency of said circuit when connected across said inductor, a gaseous electron tube connected to said capacitor and said inductor for electrically connecting said capacitor across said inductor upon conduction of said tube, a control tube, means biasing said control tube above cutoif, means including said control tube for biasing said gaseous tube below cut-on when said control tube conducts and for biasing said gaseous tube above cut-off when said control tube is cut off, a source of modulation signals for said transmitter, and means responsive to signals from said source for cutting off said control tube when said signals lower the frequency of said transmitter.

4. Frequency modulation apparatus comprising a frequency modulation transmitter, an antenna circuit for said transmitter, said circuit including an inductor, a plurality of capacitors for lowering the resonant frequency of said circuit when connected across said inductor, a gaseous electron tube associated with each of said capacitors, means connecting each said capacitor in series with its associated gaseous electron tube across said inductor, a plurality of control tubes, one connected to each of said gaseous tubes, means for normally biasing said control tubes above cutoif, means including said control tubes for biasing said gaseous tubes below cut-off when said control tubes conduct and for biasing said gaseous tubes above cut-off when said control tubes are cut off, a source of modulation signals for said transmitter, and means responsive to signals from said source for variably cutting oif said control tubes in accordance with the magnitude of a signal which lowers the frequency of said transmitter.

5. Frequency modulation apparatus comprising a frequency modulation transmitter having an assigned center frequency, an antenna circuit for said transmitter, said circuit including an inductor, a rst and a second plurality of capacitors for lowering the resonant frequency of said circuit when connected across said inductor, a first plurality of gaseous electron tubes, one of said rst tubes being connected to each of the capacitors of said first plurality, a second plurality of gaseous electron tubes, one of said second tubes being connected to each of the capacitors of said second plurality, means connecting each said capacitor in series with its associated gaseous electron tube across said inductor, a control tube connected to each of the gaseous tubes of said rst plurality of gaseous tubes, a control tube connected to each gaseous tube of said second plurality of gaseous tubes, means including said control tubes for biasing each gaseous tube below cut-olf, when it associated control tube conducts and for biasing each gaseous tube above cut-off when its associated control tube is cut off, means for normally biasing said first control tubes above cut-off and for biasing said second control tubes below cut-off, a source of modulation signals for said transmitter, and means responsive to signals from said source for variably cutting off the control tubes of said first plurality in accordance with the magnitude of said signals which lower the frequency of said transmitter below said center frequency, and for variably causing the control tubes of said second plurality to conduct in accordance with the magnitude of said signals which raise the frequency of said transmitter above said center frequency.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,077,223 Crosby Apr. 13, 1937 2,388,233 Whitaker Oct. 30, 1945 2,440,081 Fick Apr. 20, 1948 2,480,820 Hollingsworth Aug. 30, 1949 2,559,023 McCoy July 3, 1951 FOREIGN PATENTS Number Country Date 878,645 France Jan. 26, 1943 

